1. Field of the Invention
The present invention relates to a motor drive device that controls the total current that flows through a motor by controlling the duty factor of the on/off periods of a switching device connected between a power source and the motor.
2. Description of the Prior Art
A conventional example of the configuration of a motor drive device as described above is shown in FIG. 8. From unillustrated hole devices fitted around the rotor, i.e. the rotary portion, of a motor M, hole signals H1, H2, and H3 are fed respectively to between external terminals IN1 and IN2, between external terminals IN3 and IN4, and between external terminals IN5 and IN6. These hole signals H1, H2, and H3 are amplified respectively by hole amplifiers 1-1, 1-2, and 1-3, and are then fed to a signal synthesizer circuit 2xe2x80x2.
The hole signals H1, H2, and H3 respectively represent the positional relationship between the coils L1, L2, and L3 and the rotor of the motor M, and are each a sinusoidal signal synchronous with the rotation of the motor M. Moreover, the three hole signals H1, H2, and H3 are 120xc2x0 out of phase with one another.
On the basis of the signals output from the hole amplifiers 1-1, 1-2, and 1-3, the signal synthesizer circuit 2xe2x80x2 produces and outputs sinusoidal drive signals D1, D2, and D3 each having a phase 30xc2x0 leading the phase of the corresponding one of the hole signals output from the hole amplifiers 1-1, 1-2, and 1-3.
The drive signals D1, D2, and D3 output from the signal synthesizer circuit 2xe2x80x2 are fed respectively to output circuits 8-1, 8-2, and 8-3. The configuration of the output circuit 8-1 is shown in FIG. 9. The drive signal D1 fed from the signal synthesizer circuit 2xe2x80x2 is, on the one hand, fed through a switch 81 to the gate of an upper transistor (an n-channel MOS-type FET connected between external terminals P+ and OUT1) 82 and, on the other hand, inverted by an inverter circuit 83 and then fed through a switch 84 to the gate of a lower transistor (an n-channel MOS-type FET connected between external terminals Pxe2x88x92 and OUT1) 85. The switches 81 and 84 are each so configured as to be turned on when the pulse signal P output from the comparator 9, described later, is at a high level and off when the pulse signal P is at a low level.
The output circuits 8-2 and 8-3 are configured in the same manner as the output circuit 8-1, except that the node between the upper and lower transistors 82 and 85 is connected to an external terminal OUT2 in the output circuit 8-2 and to an external terminal OUT3 in the output circuit 8-3.
The external terminals OUT1, OUT2, and OUT3 are connected respectively to one end of the coils L1, L2, and L3 of the motor M. The coils L1, L2, and L3 are connected together at the other end. To the external terminal P+, a supply voltage Vcc is applied. The external terminal Pxe2x88x92 is connected through an externally fitted resistor R to ground GND. The total current that flows from the supply voltage Vcc into the motor M flows through the resistor R (of which the resistance is 1 [xcexa9] or lower) to ground GND.
A current detector circuit 6 amplifies the difference between a current specifying voltage fed in via an external terminal IN7 and the voltage (hereinafter the detected voltage) appearing across the resistor R according to the total current that flows through the motor M, and outputs the amplified difference. A comparator 9 compares the level of the signal output from the current detector circuit 6 with the level of a triangular wave output from an oscillator circuit 10, and outputs a pulse signal P that represents the result of comparison. The higher (or lower) the current specifying voltage fed in via the external terminal IN7 is relative to the detected voltage, the higher (or lower) the duty factor of the high-level periods of the pulse signal P output from the comparator 9.
The configuration described above makes it possible to control the duty factor of the on/off periods of the transistors 82 and 85 constituting the output circuits 8-1, 8-2, and 8-3 in such a way as to keep the current specifying voltage and the detected voltage equal. Thus, the total current that flows through the motor M is stabilized at the level specified by the current specifying voltage.
However, in the motor drive device described above, the switching devices connected between the power source and the motor are controlled by signals obtained by chopping sinusoidal signals with a pulse signal. As a result, the switching speed lowers near the zero-cross points of the sinusoidal signals, leading to a greater switching loss and thus lower power efficiency.
Moreover, the duty factor of the on/off periods of the switching devices connected between the power source and the motor varies only according to the error, from the specified level, of the total current that flows through the motor. Thus, distortion is inevitable in the waveform of the phase current that flows through the motor, resulting in unsatisfactory rotation characteristics such as torque ripples, wow and flutter, and rotation noise.
An object of the present invention is to provide a motor drive device that controls the total current that flows through a motor by controlling the duty factor of the on/off periods of a switching device connected between a power source and the motor and that offers improved power efficiency and rotation characteristics.
To achieve the above object, according to the present invention, a motor drive device is provided with means for producing a pulse signal having a fixed amplitude by comparing a drive signal that is a sinusoidal signal synchronous with the rotation of a motor with a predetermined high-frequency signal, and the switching of a switching device connected between a power source and the motor is controlled by the pulse signal.
This configuration prevents the lowering of the switching speed of the switching device connected between the power source and the motor near the zero-cross points of the sinusoidal drive signal. This helps minimize switching loss and enhance power efficiency. Moreover, the phase current that flows through the motor comes to have a sinusoidal waveform. This helps enhance rotation characteristics.
The motor drive device may be further provided with means for controlling the amplitude of the high-frequency signal according to a current specifying signal that indicates the level of current to be passed through the motor. The motor drive device may be further provided with a hole amplifier for amplifying the hole signal output from a hole device fitted around the rotor constituting the rotary portion of the motor, a signal synthesizer circuit for producing the drive signal on the basis of the hole signal amplified by the hole amplifier, and means for controlling the gain of the hole amplifier or of the signal synthesizer circuit according to a current specifying signal that indicates the level of current to be passed through the motor.
These configurations make it possible to vary the relationship between the amplitudes of the drive signal and the high-frequency signal on the basis of the current specifying signal. Since the maximum duty factor of the pulse signal varies according to this relationship, it is thus possible to adjust the total current that flows through the motor.
In a case where, as described above, the relationship between the amplitudes of the drive signal and the high-frequency signal is varied, it is preferable to further provide means for exercising control so that the amplitude of the high-frequency signal does not become smaller than the amplitude of the drive signal.
This configuration prevents the problem in which the switching device connected between the power source and the motor remains on in periods in which it should perform switching, with the result that the phase current that flows through the motor comes to have a stepped waveform, degrading rotation characteristics.